1. Field of the Invention
The present invention relates to a rotary drum type device for coating granular solids such as tablets, catalyst-carrying particles or the like.
2. Brief Description of the Prior Art
There have been devised and demonstrated various types of devices for coating granular solids. One is of the rotary drum type having a rotary drum consisting of a cylindrical section and side walls, the wall of the cylindrical section being perforated. One side wall has an inlet-outlet opening for feeding granular solids into the rotary drum, and the other side wall is connected to a rotary shaft which in turn is drivingly connected to an electric motor. An annular duct surrounds the perforated cylindrical section of the drum and is divided with partition walls into an upper inlet duct and a lower outlet duct. An inlet pipe and an outlet pipe or exhaust pipe are communicated with the annular duct in diametrically opposed relationship so that the hot air or the like flows from the inlet pipe through the inlet duct, the openings in the cylindrical section of the drum, the drum, the openings in the cylindrical section, the outlet duct and the outlet or exhaust pipe. As the rotary drum is rotated, a tumbling bed of granular solids is formed and sprayed with a coating liquid containing a coating agent. The coating liquid applied to the surfaces of the granular solids is dried by the hot air flowing as described above.
In the coating device of the type described, some coating liquid particles pass through the tumbling bed and are deposited on the walls of small holes in the cylindrical section of the drum and over the areas in the vicinity of the small holes. Since the hot air passes through these small holes, the coating liquid is dried, leaving layers or scales of the coating agent. As the drum is rotated, these small holes face the inlet duct so that the hot air flows into the drum through these holes and consequently the deposited layers or scales of the coating agent are separated and fall into the drum. As a result, the coated granular solids are contaminated with the separated coating agent.
In order to overcome this problem, there have been proposed various devices as disclosed in Japanese Patent Publication Nos. 49-22702, 50-38713 and 51-18397. In these devices, while the hot air is discharged through the wall of the rotary drum, it is directly fed into the drum substantially at the center thereof through an opening of one side wall of the drum, unlike the first-mentioned device in which the hot air is charged through the small holes in the wall of the cylindrical section of the drum.
In the process for coating granular solids with the coating device, it is essential that the granular solids be effectively applied with a coating liquid which is sprayed. To this end, the hot air must be used not only for drying the granular solids but also for carrying and directing sprayed coating liquid mists from the top of the drum toward the tumbling bed of granular solids, whereby the coating efficiency can be remarkably improved. However, with the prior art coating devices of the types described, such a hot air stream as described above cannot be obtained, but the hot air flows in random directions so that the sprayed coating liquid mists are spread in all directions in the drum and adhere to the whole inner wall surface of the drum, resulting in scaling.